The remains of a well house, probably medieval, are located in a dell above a bay, Porth Gwalch, in the cliffs of Holy Island. The building, known as St Gwenfaen's Well, stood above a natural east-west cleft in the rock some 1.5-2.0m deep and only the base of the western facade and the substructure set within the cleft survive.

The well house was a massive drystone structure 4.5m east-west by originally 5.5m. Internally, within the cleft, there is a chamber about 2.0m square, entered by steps leading down from the east, with a pool or bath, recessed in its western wall. The chamber has a paved floor and diagonal slab seats at the angles. Steps lead down from the chamber into the bath, which is roughly 1.2m long and 0.6m wide, with deep recessed seats on either side.

There is a larger rectangular pool in the cleft outside the west wall of the well house. It is about 1.5m across and its waters, held back by a stone slab, are at the same level as those of the bath within. The sides of the cleft are ramped here to allow access. The water flows out through a small hole in the slab into a 4.6m long paved channel, eventually finding a small pool some 35m west of the well house, above Porth Gwalch.

Sources: RCAHM Anglesey Inventory (1937), 146
NMR Site File

John Wiles 03.08.07